The invention relates to a shape measuring method for detecting the shape of a workpiece surface by interferometric measuring method, as well as to a corresponding measuring device for implementing this method.
Interferometric methods are frequently used in practical operations for the distance or also the shape measuring, wherein these methods are used to determine the distance between a point or a point array on a surface and a measuring device. Difficulties can arise in the process if the workpiece surface contains edges or gaps or if the shape to be imaged is large enough, so that it either exceeds the focal depth or the measuring range of the measuring device.
U.S. Pat. No. 5,133,601, for example, discloses a surface profile detection method which can be attributed to the white-light interferometry. The measuring device makes use of a double-beam interferometer with a measuring beam and a reference beam. Interference can be detected only if the optical path lengths between the object wave and the reference wave differ by less than the coherence length of the light that is used. The use of white light therefore results in an extremely low depth resolution of the measuring device.
U.S. Pat. No. 5,398,113 also discloses a method which can be attributed to white-light interferometry. In that case, the object is viewed with the aid of a camera through a double-beam interferometer, wherein the light source used is a polychromatic or white light source. The images recorded by the camera are interference images. The interference image changes during a relative movement of the objective and the workpiece in Z direction. Signals are thus generated at the individual pixels of the camera, which are subjected to a pixel-by-pixel digital Fourier analysis.
The digital Fourier analysis is used to detect discrete frequencies of a spectrum, wherein the discrete frequencies cannot be positioned with optional density next to each other, thus resulting in a limited depth resolution. In addition, problems can develop along the edges or shoulders of a body.
Based on this, it is the goal of our invention to provide a measuring method for detecting the height profile or the shape of a workpiece with the aid of interferometry, which method can be realized with low apparatus expenditure and achieves good measuring accuracy.